The VASI (Visual Approach Slope Indicator) and the PAPI (Precision Approach Path Indicator) are systems of lights that provide guidance information to help pilots acquire and maintain the correct approach to an airfield. They are generally located beside the runway. These lights can be seen from up to 8 kilometers (5 miles) during the day and up to 32 kilometers (20 miles) or more at night. Further information is available in the video below:

Thanks to both Pierre, and Xavier who shared this link, you can see how the events unfolded, and how – thanks to the pilot Chesley « Sully » Sullenberg, the flight officer Jeffrey Skiles, and the air traffic controller’s self-control – the flight 1549 passengers remained unscathed: (you can read the radiocommunication transcript in the bottom right corner of the video hereafter)